1. Field of the Invention
This invention relates to a radial piston pump for low-viscosity fuel and a method for assembling the pump, more particularly to a radial piston pump for low-viscosity fuel which is ideal for high-pressure delivery of low-viscosity fuel and which, owing to an improvement in its fuel paths, features high reliability and compact size, and to method for assembling the pump.
2. Prior Art
Improvement of the combustion efficiency of the internal combustion engines used in vehicles and the like is widely accepted as necessary for reducing the pollution caused by exhaust gases, as well as for preserving natural resources. An effective way of improving combustion efficiency in a gasoline engine is to promote fine atomization of the fuel spray by delivering the gasoline under high pressure. Fine atomization is also required in the case of alcohols and other such alternative fuels (referred to simply as "alcohols" in this specification) now being considered for use in internal combustion engines, since alcohols have poor cold starting characteristics.
Since the delivery pressure of 3-4 Kgf/cm.sup.2 of ordinary fuel pumps is insufficient for achieving the required degree of atomization, there is a need for a high-performance pump capable of producing a delivery pressure of several tens of Kgf/cm.sup.2.
As a type of pump suitable for this purpose, the radial piston pump is a strong candidate from the aspects of performance and efficiency. The radial piston pump is described, for example, in Japanese Patent Disclosure No. Sho 60-216081.
The prior art radial piston pumps, such as that taught by Japanese Patent Disclosure No. Sho 64-367, have generally been used as hydraulic pumps, namely as means for delivering pressurized high-viscosity oil (viscosity .gtoreq.30 cst). No problems arise regarding the performance of the prior art radial pumps so long as they are used with high-viscosity oil. The viscosity of alcohols is, however, on the order of 0.5 cst, which is extremely low.
An attempt to adapt a conventional radial piston pump for high-pressure delivery of a fuel having such low viscosity property simply by modifying the cylinder block from the rotary type to the stationary type or to the stationary cylinder type in which only the pistons reciprocate is doomed fail because the pump will not be able to maintain its performance. This because such a modification does not overcome such problems as, for example, dissolving and dilution of the grease sealed in the drive shaft bearing by the low-viscosity fuel and galling and seizing occurring between the pistons and the barrels or between the eccentric cam and the ends of the pistons.
The prior art radial piston pump is thus incapable of smoothly and stably delivering low-viscosity fuel under high pressure.
For overcoming these problems, the assignee previously developed a practical radial piston pump capable of stably pumping low-viscosity fuels such as gasoline and alcohols even at high pressures exceeding several tens of Kgf/cm.sup.2, without damage to the bearing portions or occurrence of galling and seizing at the piston sliding surfaces.
This pump is described in detail in Japanese Patent Disclosure No. Hei 3-175158 and will be explained with reference to FIGS. 1 to 4, in which it is designated by reference numeral 1.
The radial piston pump for low-viscosity fuel 1 is shown in an overall sectional view in FIG. 1, in an exploded perspective view in FIG. 2, and in a sectional view along line III--III of FIG. 1 in FIG. 3. The radial piston pump for low-viscosity fuel 1 has a housing member 2 (first pump housing member), a fixed cylinder 3 (second pump housing member), a cover 4 (third pump housing member), an intake side gasket 5, an intake se leaf valve member 6, a discharge side gasket 7, a discharge side leaf valve member 8 and a pump shaft 9. The housing member 2, fixed cylinder 3, cover 4, intake side gasket 5, intake side leaf valve member 6, discharge side gasket 7 and discharge side leaf valve member 8 are held in position by positioning pins 10 and fixed together by stud bolts 11.
The pump shaft 9 is inserted into a hole at the center of the joined members to be rotatably supported by a radial bearing 12 provided on the housing member 2, a thrust bearing 13 provided on the cover 4 and a radial bearing 14 provided on the cover 4, and is rotated through a drive pulley 15 by an external engine (not shown). The portion of the pump shaft 9 positioned within the pump shaft 9 is formed with an eccentric cam 16.
A first oil seal 17 and a second oil seal 18 provided in the housing member 2 on opposite sides of the radial bearing 12 prevent the low-viscosity fuel, which is a solvent, from diluting the grease sealed in the radial bearing 12. A fuel intake port member 19 is mounted on the housing member 2 and the interior portion in communication with the intake port member 19 is formed with intake passages 20 and radially formed annular chamber inlets 21, and with a center cavity 22. The number of intake passages 20 and annular chamber inlets 21 is the same as the number of cylinder holes 23 and cap-shaped pistons 24 (both explained below) provided in the fixed cylinder 3 (five in the example illustrated in FIG.3).
The annular chamber inlets 21 are inlet passages for feeding low-viscosity fuel into an annular chamber 33 (explained later) and supplying it to the cylinder holes 23, and are used for lubrication.
The cylinder holes 23 (five in the illustrated example) are formed to extend radially through the fixed cylinder 3 at regular intervals. A cap-shaped piston 24 is accommodated in each fixed cylinder 3 to be free to reciprocate therein. In each fixed cylinder 3, the piston 24 is pressed radially inward by a piston spring 26 inserted between itself and a plug 25 screwed into the cylinder holes 23. As the pistons 24 slidingly contact the eccentric cam 16 of the pump shaft 9 at their head portions, they reciprocate as the pump shaft 9 rotates. This reciprocation produces a pumping action for intake and delivery of the low-viscosity fuel.
The position of contact between the pistons 24 and the eccentric cam 16 is approximately at the center of the fixed cylinder 3.
An intake side passage 27 and a discharge side passage 28 are formed in the fixed cylinder 3 on opposite sides of each cylinder hole 23 so as to communicate therewith. The intake side passage 27 and the discharge side passage 28 communicate with a pressurization chamber 29 (FIG. 3) formed between the piston 24 and the plug 25.
The intake side leaf valve member 6 is formed with intake valves 30 constituted as tongue-shaped springs (FIG. 2). The range of movement of the intake valves 30 is regulated by generally rectangular stops 31 disposed in the fixed cylinder 3 so as to selectively enable fuel intake through the intake valves 30 and the intake side passages 27 in accordance with the reciprocation of the pistons 24.
A center cavity 32 of the fixed cylinder 3 and the center cavity 22 of the housing member 2 together form the annular chamber 33 for fuel intake and lubrication.
The cover 4 has discharge passages 34 which communicate with the discharge side passages 28 of the fixed cylinder 3 and the discharge passages 34 communicate with a discharge port member 36 through a collection groove 35 (FIG. 2).
The discharge side leaf valve member 8 is formed with discharge valves 37 constituted as tongue-shaped springs (FIG. 2). The range of movement of the discharge valves 37 is regulated by generally rectangular stops 38 disposed in the cover 4 so as to selectively enable fuel discharge through the discharge side passages 28 and the discharge valves 37 in accordance with the reciprocation of the pistons 24.
The cover 4 is provided with a third oil seal 39 for preventing the grease sealed in the radial bearing 14 from being diluted by the low-viscosity fuel. Moreover, the third oil seal 39 and the second oil seal 18 close the annular chamber 33 off from the exterior at the ends of the pump shaft 9.
As shown in FIG. 4, the cover 4 is further provided with a relief valve 40 that communicates with the discharge passages 34 and the discharge port member 36. A return port 41 communicating with the annular chamber 33 is formed near the opening of the relief valve 40 for returning low-viscosity fuel to the annular chamber 33 when the pressure of the low-viscosity fuel becomes abnormally high.
As shown in FIG. 1, low-viscosity fuel is supplied to the intake port member 19 from a fuel tank 42 by a feed pump 43 and the pump action of the pistons 24 reciprocated by sliding contact with the eccentric cam 16 driven by the pump shaft 9 delivers the low-viscosity fuel to an injector (or a common rail).
To be more specific, when the pistons 24 move centripetally during the intake stroke, the intake valves 30 open and the discharge valves 37 close, so that fuel is drawn into the pressurization chambers 29 through the intake port member 19, the intake passages 20, the intake valves 30 and the intake side passages 27. On the other hand, when the pistons 24 move centrifugally during the discharge stroke, the intake valves 30 close and the discharge valves 37 open, so that fuel is discharged from the pressurization chambers 29 through the discharge side passages 28 and the discharge valves 37 to be delivered under high pressure to an injector 44 through the discharge port member 36.
The radial piston pump for low-viscosity fuel 1 described in the foregoing nevertheless has a number of drawbacks deriving from the three-body structure of the pump housing (the housing member 2, the fixed cylinder 3 and the cover 4 joined by the stud bolts 11). Specifically, the pump consists of a large number of components, is therefore difficult to assemble with highly precise overall axial alignment, is susceptible to wobbling of the pump shaft 9, is likely to develop performance and reliability problems as a radial piston pump for low-viscosity fuel, and is expensive to manufacture.
FIG. 5 is an overall sectional view of a prior-art radial piston pump for low-viscosity fuel 50 having a four-body structure. Components in FIG. 5 which are similar to those in FIGS. 1 to 4 are assigned the same reference numerals as those in FIGS. 1 to 4 and will not be explained further here.
The radial piston pump for low-viscosity fuel 50 comprises a drive-side housing member 2A and a driven-side housing member 2B which together correspond to the housing member 2, a fixed cylinder 3, a cover 4, an outer cover 51, an intake side leaf valve member 6 located between the driven-side housing member 2B and the fixed cylinder 3, a discharge side leaf valve member 8 located between the fixed cylinder 3 and the cover 4, and a nonmagnetic material partition 52 disposed between the drive-side housing member 2A and the driven-side housing member 2B.
The outer cover 51 covers the driven-side housing member 2B, the fixed cylinder 3 and the cover 4, which three components are joined by stud bolts 11.
The pump shaft 9 is divided into a drive-side pump shaft 9A and a driven-side pump shaft 9B which are located on opposite sides of the partition 52 and inked by a magnetic coupling 53 straddling the partition 52. The magnetic coupling 53 comprises a drive-side magnet 54 attached to the drive-side pump shaft 9A and a driven-side magnet 55 attached to the driven-side pump shaft 9B by a bolt 56.
The drive-side housing member 2A and the driven-side housing member 2B are joined by housing bolts 57.
The remainder of the structure for achieving pump function is substantially the same as that of the radial piston pump for low-viscosity fuel 1 of FIG. 1, except that the pistons 24 are accommodated in cylinder liners 58.
As indicated by the solid-line arrows in FIG. 5, the fuel paths are established such that fuel passes through intake passages 20, an annular chamber 33 and intake side passages 27 to the pistons 24, where it is subject to pump action, and is then discharged under pressure to an injector 44 through discharge side passages 28 and discharge passages 34.
The driven-side pump shaft 9B is provided with washers 60, 61, 62, 63 and 64 and with a first roller bearing 65, a third roller bearing 66 and a second roller bearing 67, and opposite ends of the driven-side pump shaft 9B are supported by a first bearing bush 68 and a second bearing bush 69. The washers 60-64 prevent wearing of the side surfaces of the bushes 68 and 69, the rollers 65-67 enhance wear resistance by lowering the sliding speed, and the bushes 68 and 69 increase wear resistance and reduce thermal deformation.
The pump housing of the radial piston pump for low-viscosity fuel 50 constituted in the foregoing manner thus has a four-body structure consisting of the drive-side housing member 2A, the driven-side housing member 2B, the fixed cylinder 3 and the cover 4 fixed together by the stud bolts 11 and the housing bolts 57.
It therefore has all of the various shortcomings of the radial piston pump for low-viscosity fuel 1 shown in FIG. 1. In addition, since the cylinder liners 58 and the pistons 24 are squeezed between the driven-side housing member 2B and the cover 4, the cylinder holes 23 may deform depending on the degree of tightening of the stud bolts 11.
FIG. 6 is an overall sectional view of a prior-art radial piston pump for low-viscosity fuel 70 which is of the type not having the outer cover 51. Being configured similarly to the radial piston pump for low-viscosity fuel 1 of FIG. 1., the radial piston pump for low-viscosity fuel 70 has a housing member 2, an intermediate block 2C, a fixed cylinder 3, a cover 4, an intake side leaf valve member 6 and a discharge side leaf valve member 8.
The remaining portions are substantially the same as those of the radial piston pump for low-viscosity fuel 1, except that fuel can be recirculated by overflow from a recirculation passage 59 communicating with an annular chamber 33 for intake and lubrication.
The pump housing of the radial piston pump for low-viscosity fuel 70 thus also has a four-body structure, in which the housing member 2, intermediate block 2C, fixed cylinder 3 and cover 4 are joined by stud bolts 11. It therefore involves the same problems as the radial piston pump for low-viscosity fuel 1 of FIG. 1 and the radial piston pump for low-viscosity fuel 50 of FIG. 5.
Other prior-art radial piston pumps for low-viscosity fuel are taught by Japanese Patent Disclosure No. Hei 4-86377, Japanese Utility Model Disclosure No. Hei 4-103265 and Japanese Patent Disclosure No. Hei 5-256252. Since the fuel intake and discharge ports are not in the same plane in the direction perpendicular to the axis of the pump shaft in these pumps, however, they are troublesome to hook up and assemble. In addition, since not all of the fuel sucked into these pumps is used for lubrication and cooling of the bearings and other friction points, the lubrication and cooling effect is inadequate.
They also have the disadvantage that the axial length of the pump shaft is increased when they are equipped with a cylinder type magnetic coupling.
The prior-art magnetic coupling type radial piston pump for low-viscosity fuel also have shortcomings in that a high degree of skill is required for ensuring that the various parts are assembled in the proper positional relationship and in that high-precision axial alignment is hard to achieve.
This invention was accomplished in light of the foregoing problems of the prior art and has one of its objects to provide a radial piston pump for low-viscosity fuel which achieves a reduction in the number of pump housing members irrespective of whether or not an outer cover is incorporated, enables high-precision overall axial alignment, prevents pump shaft wobble, improves performance and reliability and achieves a reduction in cost.
Another object of the invention is to provide a method for assembling a radial piston pump for low-viscosity fuel which enables reduction of overall pump size, improvement of pump performance, easier pump assembly and improvement of pump shaft axial alignment.
Another object of the invention is to provide a radial piston pump for low-viscosity fuel enabling effective lubrication and cooling of the pump shaft bearings and other friction points, and a method for assembling the same.
Another object of the invention is to provide a radial piston pump for low-viscosity fuel which is easy to hook up to external tubing, and a method for assembling the same.